Thanks for writing to the forum!
times an allergy starts in reaction to heat or cold
. What you have is probably cholinergic urticaria. Exercise, hot baths, tight clothes, spicy food, emotional stress and fever can all cause this. Once this is identified, avoiding the reason can take care of the problem. Sometimes rapid cooling of the body can prevent an attack. Antihistamines also help.
It is difficult to comment beyond this at this stage.